It is well established that photosensitive resin compositions may be used as photoresists in the manufacture of printed circuit boards, and the like. Currently, dry film photoresists dominate the market. However, fine line imaging capability of such dry films is limited, because of their inability to conform completely to microscopic variations in the substrate surface (i.e., the copper surface) during lamination. Also, very fine lines tend to be poorly resolvable due to the thickness of such dry film photoresists when combined with the cover sheet (that separates the artwork from the photoresist) during exposure.
As a result, the industry has begun to develop liquid applied photoresist systems. To effectively compete with dry film photoresists, the industry has sought to develop liquid applied photoresists that have the advantages of dry film photoresists (e.g., contact imageability and durability), without their disadvantages (e.g., limitations on fine line imaging and surface contact/adhesion to substrates).
Liquid applied photoresists were initially organic solvent-based systems and solvent developable. Recently, however, in response to health, safety and environmental concerns, the need to reduce and/or remove organic solvents from the preparation of photoresist compositions and photoresist films has become a priority. Waterborne aqueous-developable photoresist compositions have, therefore, been developed to address this need.
In developing waterborne photoresist compositions, the industry has sought to obtain compositions and resulting films that provide the advantages of solvent-based systems. With waterborne photoresist systems, however, the challenge has been to provide compositions that retain their stability, while still being able to form versatile photoresist films. Since stabilization of waterborne photoresist systems has typically required the use of certain additives that can degrade photoresist performance, it is believed that the most versatile photoresist films may be obtained when such additives are limited.
Waterborne photoresist compositions are essentially aqueous emulsions of water-insoluble compounds. These emulsions require stabilization to prevent phase separation of the insoluble compounds. To be effective, these emulsions must be both storage stable and shear stable.
Photoresist emulsion compositions typically require storage stability of at least about 6 months in order to account for formulation time, shelf-time, and shipping time to customers. Photoresist emulsion compositions that are not storage stable for at least about 6 month run the risk of phase separation during storage, shipping, and/or use. Should phase separation occur, the emulsion would become completely unusable, significantly impacting upon manufacturing costs. Therefore, the industry typically demands that photoresist emulsions have a storage stability of at least about 6 months.
Stabilization is also necessary due to the processing requirements and intended application of these photoresist emulsions. These aqueous emulsions are frequently subject to high shear conditions in the manufacturing process, especially in the case of direct emulsification of previously synthesized polymers. In many cases, the photoresist emulsions are then concentrated by evaporation of water and/or organic solvents, often under vacuum and with heat and mechanical shear stress (e.g., wiped filmed evaporators). In cases where organic solvents are emulsified along with other water-insoluble components, the evaporation process itself can be mechanically stressful, as solvent vapors must pass through the surfactant boundary, thus disrupting the stabilizing forces.
Furthermore, photoresist emulsions are frequently formulated with other additives using high shear to mix the components. Additives such as water-dispersible thickeners and organic co-solvents can, however, destabilize emulsions and increase their sensitivity to mechanical and/or thermal stress. Although the exact mechanism is not clear, it is believed that additives tend to make emulsions more susceptible to coagulation.
Application methods may also require high shear stability, especially with regard to spray- and roll-coating processes. Therefore, it is necessary that photoresist emulsion compositions do not degrade or coagulate under high shear conditions.
Emulsifiers and other surfactants are commonly used to overcome emulsion and shear stability problems in aqueous emulsion photoresist systems. However, high levels of surfactants are often required to provide sufficient stability, which can negatively impact upon film formation and the performance characteristics of dried films.
The use of high levels of surfactants can, for example, result in foam stabilization problems in the emulsion during high shear processes, which can lead to the formation of bubbles or pin holes in the final film product. Excessive surfactant levels can also lead to adhesion failure in dried films at the substrate/coating interface, due to their tendency to migrate to the interface surface. Excessive levels of surfactant can further lead to increased sensitivity of the dried films to water or humidity, due to their hydrophilicity. Moreover, an increase in the hydrophilic character of photoresists (by, e.g., the use of increasing levels of surfactants) tends to both increase the developing rate to impractically high speeds and reduce the resistance of the photocured regions to attack by the developer. Therefore, the use of high levels of surfactants to achieve emulsion stability for waterborne photoresist systems is not favored.
Recently, practitioners have turned their focus to stabilizing aqueous photoresist emulsion systems by neutralizing the acid functional polymer of the photoresist emulsion with base. For example, in U.S. Pat. No. 5,045,435, a waterborne, alkali-developable, photoresist coating composition is disclosed which is said to comprise a multifunctional monomer, a photoinitiator, and a latex of a water insoluble carboxylated acrylate polymer, in which at least 25% of the acid groups of the latex are neutralized. It is stated that a stable photoresist emulsion may be prepared by neutralization at levels greater than at least 25% with a base such as ammonia, and more typically at about 30% to 60%, to achieve desired stability and viscosity.
However, it has been found that while high degrees of neutralization, as required in U.S. Pat. No. 5,045,435, may impart desired Theological properties such as higher viscosity, thereby contributing to emulsion stability, this is achieved by sacrificing film properties. More specifically, increased levels of neutralization tend to reduce chemical resistance of the dried films, since the neutralizer in reacting with the acid functional polymer produces water soluble salts that are not all removed during drying, and become sites for potentially damaging chemical attack. Furthermore, it has been found that at high levels of neutralization, these emulsions become so viscous that they require dilution, to be suitable for coating processes, even at relatively low solids levels. Moreover, it is believed that high levels of neutralization can cause destabilization of these emulsions.
More recently, practitioners have made efforts to overcome these problems by reducing the levels of neutralization, thereby necessitating the use of other additives, emulsifiers and/or other surfactants, to compensate for the loss of rheological properties attendant with high neutralization.
For example, thickeners are well known in the art and commonly used for their ability to adjust rheology of pre-formed emulsions by increasing viscosity (i.e., thickening the emulsion). Although associative thickeners are predominantly used, non-associative thickeners may also be used. Associative thickeners, when used for adjusting rheology of photoresist emulsions, are typically used in the range of about 0.1 wt. % to about 2.0 wt. % (on a weight basis of thickener solids to total solids in the emulsion).
U.S. Pat. No. 4,426,485 discloses that certain associative thickeners are useful in enhancing particle wetting of materials being added to a resin system (i.e., acting as surfactants).
U.S. Pat. No. 5,364,737, employs associative thickeners in combination with polymer neutralization of less than or equal to about 20% for a photoresist composition. The patent claims to the use of a polyether polyurethane associative thickener in amounts of from about 1 wt. % up to about 40 wt. %. It is stated in Example 3 that the use of a polyether polyurethane associative thickener at about 2.78 wt. % (on a weight basis of thickener solids to total solids in the emulsion) results in a film with slight ribbing; in Example 4, it is stated that use of about 5.56 wt. % a polyether polyurethane associative thickener results in a film with no ribbing.
U.S. Pat. No. 5,364,737, more specifically, discloses a waterborne photoimagable composition comprising a partially neutralized (.ltoreq.20%) latex binder polymer having carboxylic acid functionality, ethylenically unsaturated monomers, a photoinitiator, and at least about 1 wt. % to about 40 wt. % of a polyether polyurethane associative thickener. The associative thickener is used for the dual purpose of adjusting viscosity and as a surfactant to emulsify the monomer fraction, thereby helping to stabilize the latex emulsion of the binder polymer. The associative thickener is, thus, used in, (1) forming a waterborne emulsion of the latex binder polymer, monomers, photoinitiator, and conventional additives and (2) improving the rheology of the stabilized emulsion.
In comparison with the levels of associative thickener used by skilled artisans to adjust rheology of photoresist emulsions, U.S. Pat. No. 5,364,737, requires relatively high levels of about 1 wt. % up to about 40 wt. % of associative thickener to substantially reduce the level of neutralization of the latex polymer. These required high levels of associative thickener, however, have their own set of disadvantages relating to the nature and level of solids in the emulsion, during photoresist emulsion formation, and to the final film properties in the dried film.
The term "solids" when referring to photoresist emulsions is meant to include all of the non-volatile components of the emulsion that remain in the film when dried. The "solids" in a photoresist emulsion will, therefore, comprise the non-volatile portions of: latex polymers, monomers, photoinitiators, thickeners, surfactants, neutralizing base, and any other non-volatile additives.
The required presence of associative thickeners in emulsion formation of the '737 patent places limits not only on the ultimate total solids content of the emulsion due to viscosity effects, but also on the proportionate distribution of the active photoimageable components (e.g., polymer, monomer, photoinitiator, dye) to the inactive non-volatiles (e.g., thickener, surfactant, defoamer, flow control agent) in the emulsion.
Limiting the total solids content of the emulsion, in turn, limits film thickness that can be reasonably achieved with various application techniques including spray coating, dip coating and roll coating. These application techniques cannot typically tolerate extremely viscous fluids since such materials tend not to flow and level properly after contact with the substrate, or may encounter difficulty in actually transferring to the substrate.
With respect to film thickness, in a given application process optimized for a material with a certain viscosity range, a one mil thick wet coating might be expected to provide a 0.25 mil thick dry coating in the case of a 25% solids emulsion and a 0.5 mil thick dry film in the case of a 50% solids emulsion, based on total solids in the emulsion. In short, the higher the solids content of the emulsion, the thicker will be the resulting film. Perhaps, more importantly, though is the versatility that is achieved with a higher solids content, because such versatility allows for tailoring the solids/rheology of the emulsion to differing requirements of manufacturing equipment and desired products. This is particularly important in achieving fine line resolution (i.e., sharpness) in a film of a given thickness.
Thus, there exists the need for stable, viscosity-adjustable waterborne photoresist emulsions in which the total amount of solids in the emulsion can be tailored to specific needs, including very low solids (&lt;20%) to very high solids (45-60%).
The presence of relatively high levels of associative thickeners in photoresist emulsions, as in U.S. Pat. No. 5,364,737, tends to promote "tack" and/or "blocking" in the dried film. In the preparation of photoresists, it is usually necessary to bring the dried photoresist film into contact with a phototool (or "photographic" negative) through which it is exposed to actinic light. "Tack" refers to stickiness without transfer from the film, of some of the photoimageable composition when the film is brought into contact with a negative. "Blocking" refers to sticking with transfer between two surfaces of the same material, encountered during stacking of uncured photoresist panels. It is, therefore, important for photoresist films to exhibit anti-tack and anti-blocking, since stickiness and/or transfer of photoimageable composition to the negative can at least necessitate cleaning of the negative, if not destroy it completely.
Another problem with prior art photoresist emulsion materials is that, following coating onto a substrate and drying, they usually cannot be stacked for compact storage. Although the prior art has addressed anti-tack, with respect to contact imageability of negatives by the use of slip aids, the issue of stackability of photoresists has remained unresolved.
During processing of photoresists, it is often necessary that the coated substrate photoresists, such as coated circuit boards, be taken off-line and stored. This may be due to work stoppages, for example, because of equipment failures, personnel situations, holidays, or any other curtailment of the production line.
In the past, coated substrate photoresists have not been able to be stacked because the pressure exerted on multiple layers of coated photoresists of the prior art have exhibited blocking. This blocking is believed to be caused by the presence of relatively high levels of thickener, especially associative thickeners, and certain surfactants, in the formation of the photoresist emulsions that remain as inactive non-volatiles in the dried film. It is, therefore, believed that stackability can also be achieved by limiting the amount of thickeners, especially associative thickeners, and certain surfactants in the formation of photoresist emulsions.
Consequently, there exists a need for photoresist emulsion compositions, and processes for making such compositions, which meet the requirements for a waterborne, liquid applied contact imageable photoresists that provide storage and shear stable emulsions having high solids and low viscosity while minimizing the need for neutralization of the acid-functional polymer latex and minimizing the use of additives such as thickeners and surfactants. There is a further need for such photoresist compositions which when coated and dried onto a substrate material exhibit sufficient anti-blocking properties to provide photoresist substrates that are capable of being stacked.